In a microscope, a processing method (photolithography method), and an optical recording method using laser light, spatial resolution is limited by the diffraction limit. That is, laser light can be narrowed down only to a space of an extent of a wavelength range due to wave nature of light. Therefore, the limit of the spatial resolution of observation, processing, and recording by laser light is about half size of the wavelength. For example, in a conventional microscope, once the laser wavelength and numerical aperture of an object lens are determined, the spatial resolution cannot be improved more than a certain value. In this way, observation performance of a microscope, density of optical recording, processing resolution of a semiconductor or the like are limited by the wavelength of laser light. In order to improve the spatial resolution, it is necessary to reduce the laser wavelength.
The spatial resolution of a microscope can be improved by using nonlinear optical effects, such as two-photon absorption and multiphoton absorption. However, in order to excite the fluorescence in a visible region by the multiphoton absorption etc., near-infrared light with a long wavelength must be used. As a result, the spatial resolution of a microscope will be almost the same as a case of usual visible region excitation.
Further, there is a laser microscope disclosed that can improve the spatial resolution without shortening the laser wavelength (patent document 1). This laser microscope uses the nonlinear optical effect generated by saturation of fluorescence to observe the fluorescence. Therefore, a fluorescence microscope with high spatial resolution can be realized without reducing the laser wavelength.
There is a fluorescence microscope disclosed that uses nonlinear effect produced by a donor molecule and an acceptor molecule. As this fluorescence microscope uses the donor molecule and the acceptor molecule, fluorescence occurs from a donor molecule by two-photon absorption or multiphoton absorption. This improves the spatial resolution.    [Patent Document 1] International Patent Publication WO2006/061947    [Non-patent document 1] “Nonlinear fluorescence through intermolecular energy transfer and resolution increase in fluorescence microscopy” Andreas Schonle et al., Ann. Phys. (Leipzig) 8 (1999) 2, 115-133    [Non-patent document 2] “Two-and multiphoton excitation of conjugate-dyes using a continuous wave laser” Pekka E. Hanninen et al., Optics Communications 130 (1996) 29-33